The immunocompetence of patients with astrocytomas and glioblastomas was studied using the in vitro lymphocyte blast transformation to mitogens PHA and CON A. Plasma from 30 to 31 patients with glioblastoma multiforme demonstrated plasma inhibition of autologous and normal lymphocytes. The inhibition of autologous lymphocytes was always greater than inhibition of normal lymphocytes. Thus, a strong plasma inhibitory factor is demonstrated in patients with glioblastoma. This same inhibition was not demonstrated in 10 astrocytoma (Grade I and II) patients. The degree of plasma inhibition correlated with the clinical condition of the glioblastoma patient. Those patients with less than 20 percent inhibition were well, those with 50 percent to 80 percent inhibition had gross neurological deficits and those with more than 80 percent inhibition were invalids or terminal.